Essential Physics II
E 英語で物理学の エッセンス II
Lecture 10: 30-11-15 News Schedule change:
Monday 7th December (12月7日) NO CLASS!
Class 11/26 homework: 12/14
This week’s homework: 12/14 So far... We have seen: Electric fields, E¯ , are created by charges. ¯ kq qenclosed E ¯ E¯ dA¯ = E = 2 rˆ r · ✏0 I
... and charges feel a force in electric fields: F¯12 = qE¯
Magnetic fields, B ¯ , are created by moving charges. ¯ ¯ B µ0 Idl rˆ ¯ dB¯ = ⇥ B dr¯ = µ0Iencircled 4⇡ r2 · I ... and moving charges feel a force in magnetic fields: F¯ = qv¯ B¯ ⇥ Question...
q E¯ This tells us how charges interact with electric and magnetic fields B¯
But how do electric fields and magnetic fields interact with each other? Induced Currents In 1831...
Michael Faraday
Joseph Henry discovered: electric currents, I , arise (start) in circuits I in a changing magnetic field. B¯
This current is the induced current. Induced Currents 4 simple experiments: Experiment 1: moving a magnet near a coiled (loops) circuit
circuit has no battery (or power source)
move the magnet toward circuit and a current flows.
move the magnet away from circuit and an opposite current flows. Induced Currents 4 simple experiments: Experiment 1: moving a magnet near a coiled (loops) circuit
No movement = no current v =0 I =0 Induced Currents 4 simple experiments: Experiment 1: moving a magnet near a coiled (loops) circuit
Move the magnet and a current starts in the loop. v =0 I =0 6 6 (but no battery!) Induced Currents 4 simple experiments: Experiment 1: moving a magnet near a coiled (loops) circuit
Move the magnet faster and current increases. Induced Currents 4 simple experiments: Experiment 1: moving a magnet near a coiled (loops) circuit
Reverse direction of magnet current reverses direction Induced Currents 4 simple experiments: Experiment 2: moving a coiled circuit near a magnet
It doesn’t matter if the magnet moves or the circuit. Only relative motion is important. Induced Currents 4 simple experiments: Experiment 3: moving a current near a circuit Replace magnet with 2nd circuit
battery I
Steady current, I Circuit moves Induced current creates B¯ field. starts Induced Currents 4 simple experiments: Experiment 4: varying current in one circuit
When neither circuits moves; no induced current
Brief (short time) But, open switch current drops in left circuit current induced I 0 in right circuit ! B 0 ! Brief current induced Close switch in current increases in right circuit left circuit 0 I 0 B (opposite direction) ! ! Induced Currents B ¯ changes because ...
magnet moves towards circuit
circuit moves towards magnet
circuit creating magnetic field moves
magnetic field increases / decreases Induced Currents
Why B ¯ changes is not important.
A current appears in a circuit feeling a changing magnetic field.
This is electromagnetic induction. Induced Currents Quiz
Which will cause an induced current in a coil of wire?
(A) A magnet resting near the coil.
(B) The constant field of the Earth passing through the coil.
(C) A magnet being moved into or out of the coil.
(D) A wire carrying a constant current near the coil. The EMF Because the magnitude of current depends on the wire material, when describing induced current we talk about EMF.
EMF, : electromotive force E Potential difference ( V ) needed to cause a current to flow.
= V = E¯ dr¯ = IR (Ohm’s Law) E circuit · I lecture 7 around circuit material’s resistance; how easy it is to create current The EMF can be: e.g. a battery e.g. the induced EMF from induction
Changing B ¯ field creates an EMF. Magnetic flux
For electromagnetic induction, the surface is the inside of the circuit.
B ¯ is stronger closer to the magnet. Flux increases through circuit when magnet is close.
For a flat surface and uniform field: = B¯ A¯ = BAcos ✓ B ·
Unit: Tm2 or Wb (weber) Magnetic flux Quiz
A circular loop with 5.0 cm diameter makes an angle of 30 to uniform magnetic field, B¯ = 80mT . | | What is magnetic flux, B ?
4 (A) 1.6 10 Wb = B¯ A¯ = BAcos ✓ ⇥ B · 3 2 2 3 = (80 10 )⇡(2.5 10 ) cos(30 ) (B) 1.7 10 Wb ⇥ ⇥ ⇥
4 (C) 1.4 10 Wb ⇥
(D) 1.3Wb Magnetic flux Magnetic flux through loop next to current carrying wire.
B ¯ field encircles wire. Points into the page in the loop: B¯ dA¯==BdABA · µ I Field around a wire: B = 0 (lecture 8) 2⇡r varies with distance
a+W µ I µ Il a+w dr = BdA = 0 ldr = 0 B 2⇡r 2⇡ r Z Za Za µ Il a + w = 0 ln 2⇡ a ✓ ◆ Faraday’s Law
Using magnetic flux, B , and the EMF, , we can write: E
Faraday’s law
d B change in magnetic flux = E dt through circuit area induced EMF
The induced EMF opposes the flux change.
Describes electromagnetic induction in circuits. (more general form later) Faraday’s Law
d = B E dt